A large number of metal matrix composite materials are known which contain, as a rule, reinforcing elements in the form of powdery particles or short or long fibers. These elements are partly embedded directly in the metal matrix or bonded to the metal matrix, forming a sandwich structure.
Furthermore, in aviation, another composite material which is also referred to as “fiber metal laminate” (FML) is used, the construction of which is described, for example, in the article by J. W. Gunnink, “GLARE: An Improved Damage Tolerance Material”, Proceedings of the 19th European Conference on Materials for Aerospace Applications, Munich, Germany, Dec. 6-8, 2000. The glare material is a sandwich construction and consists of a number of alternating layers of aluminum and glass fiber prepregs, a “prepreg” being a pre-impregnated non-crimped fiber fabric. In the case of the prepreg material used here, the glass fibers are aligned in two mutually perpendicular directions, the fiber directions usually being referred to as 0° and 90°. The number of fibers in the 0° direction and the 90° direction may be the same or different.
The total thickness of the glare material depends, in particular, on the structural requirements. The aluminum layers typically have a thickness of 0.2-0.5 mm and the prepreg layers a thickness of about 0.125 mm. On the other hand, the minimum thickness of the glare material is, for example, 0.525 mm. For practical applications, however, a thickness of 0.85 mm is preferred. Moreover, the glare material has already been produced with a thickness of up to 40 mm. The laminated composite is laminated with an adhesive, typically a duromer epoxy resin.
The advantage of the glare material lies, in particular, in its potential to save weight, so that it is used mainly in aviation, for example, as a skin or fuselage material. In addition, the material has a high damage tolerance. In particular, it offers improved resistance to crack formation, crack propagation and fracture toughness (crack resistance) in comparison to the aluminum aviation materials for pressure cladding the fuselage. By these means, the service life is improved and, with appropriate dimensioning, the weight is reduced.
However, it is a disadvantage that, because of its sandwich construction from different materials, the glare material has a low shear strength, so that the danger of delaminating is very high. This danger is increased by the adhesive used for bonding the sandwich construction, since the adhesive has only a certain, limiting strength. Consequently, in the case of shear or thrust forces, the load-carrying capacity of the material is limited. As a result, faces of holes, for example, are not stable. This means that holes, which are provided in the material, fray or extend under load, so that the shape of the holes is altered in an undesirable manner. Moreover, because of the sandwich construction described, the glare material is moisture-sensitive and the use of different types of material makes it difficult to recycle the composite material.
It is a further disadvantage that the stiffness of the material in certain directions is limited because of the special orientation of the fibers. Moreover, the appreciable anisotropy of the strength properties greatly limits the design freedom (“tailoring”).
Moreover, it is disadvantageous that the classical further processing of the material into a semi-finished product, for example, by means of shaping, drilling, heat treating, clamping, welding, forging, etc. is extremely limited, since the ceramic fibers (such as glass, carbon, boron or silicon carbide fibers) are not suitable for the integral process of future construction. Moreover, the glare material cannot be processed by bonding; this is the case for T joints (such as skin-stringer connections) as well as for butt joints.
It is therefore an object of the present invention to provide a composite material, which does not have the disadvantages named above and the strength properties of which are at least similar to or better than those of known composite materials and which can be produced easily and inexpensively and, moreover, can be recycled readily.
This objective is accomplished by a metal fiber-reinforced composite material, in which metal layers and reinforcing layers which are connected to one another alternate, the invention composite material being distinguished by the fact that the reinforcing layers contain fibers of a high-strength, metallic material. These reinforcing layers are disposed in the form of a loose structure between the metal layers, in order to create a material excess of fibers in the reinforcing layers, so that the fibers in the composite material, welded by a thermomechanical process, are extended because of the excess of material.
By introducing high strength, metallic fibers in a sufficient amount, reinforcement is introduced directly into the composite material. At the same time, the metallic inert fibers ensure compatibility with the usual manufacturing and processing procedures for such composite materials (such as rolling, extruding, shaping, clamping, forging or bonding), so that such a composite material can be produced considerably more easily and inexpensively than known composite materials. Because of the compatibility of the fibers, the composite material is furthermore distinguished by less serious structural defects with respect to the E modulus or delamination. Moreover, because of the compatibility of the fibers, recycling is facilitated.
Advisably, the layers of metal consist of a light metal alloy. The alloy has the advantage that the inventive composite material has great potential for saving weight and therefore is very suitable for lightweight construction.
Advisably, the fibers consist of an austenitic or precipitation hardened stainless steel or of a nickel-based alloy. Likewise, a metal alloy can be used, the strength of which is greater than that of the composite material and which is not permanently damaged by a thermomechanical treatment at temperatures of up to a maximum of 600° C. The fibers may, for example, be Ti, TiAl, FeAl, NiAl or Co wires. The materials mentioned have the advantage that they have a very good strength to tenacity ratio, so that semi-finished products, which are produced from the inventive composite material, have a high strength. Moreover, particularly the stainless steels have a neutral corrosion behavior when connected to aluminum or magnesium.
Advantageously, the loose structure is braided, woven or knitted from the fibers. In this way, it is possible to ensure with relatively simple means that a sufficient material excess of fibers is present within the reinforcing layers, so that the loose structure is elongated during the welding of the composite material by rolling, because of the material excess of fibers and not because of their own extensibility.
Pursuant to a further embodiment, the loose structure is a net-like, jumbled or spun structure, produced from the fibers. This embodiment has the advantage that it can be produced particularly easily.
The object of the invention furthermore is accomplished by a method for producing a metal fiber-reinforced composite material, where the metal layers and reinforcing layers are disposed alternately in a sandwich structure, the reinforcing layers containing fibers of a high strength, metallic material, which are placed in the form of a loose structure between the metal layers, in order to create a material excess of fibers in the reinforcing layers and owing to the fact that the sandwich structure is bonded by a thermomechanical process, the fibers extending because of the excess of material and the sandwich structure thus bonded.
This method is particularly distinguished by the simplicity of the process, so that such metal fiber-reinforced composite materials can be produced easily and inexpensively. Furthermore, it is advantageous that the metal fibers are compatible with typical manufacturing and processing procedures, so that the inventive, composite material can be processed further easily into profiles, sheets, semi-finished products, etc.
Advantageously, because metallic fibers of high-strength have been incorporated, the strength and fatigue, but also the crack, propagation and creep behavior of such profiles, sheets, semi-finished products, etc. are improved appreciably.
Preferably, the thermomechanical process is a hot or cold rolling process. Consequently, the method is flexible, since the bonding of the sandwich structure is not limited to a particular process.
Furthermore, it is advisable to provide the metal layers with a solder alloy before bonding of the sandwich structure by means of rolling, the solder alloy preferably being an AlSi12 alloy. In addition, other aluminum solder alloys, which can be plated by rolling, can be used. The diffusion and hot-press welding behavior of the metal layers, which generally is not good because of the very stable aluminum oxide (Al2O3) covering layers, which form on the surface of the metal layers, can be overcome by such a solder alloy.
It is particularly advantageous if the metal layers are bonded before they are actually processed or arranged into a sandwich structure. This is a simple and effective procedure for providing the metal layers with the solder alloy. Furthermore, the use of metal layers, which are provided with a solder alloy, proves to be advantageous particularly when the sandwich structure is heated to a temperature of below approximately 600° C. before it is rolled, so that the solder alloy melts and causes the sandwich structure to bond together. In this way, a stabilization of the sandwich structure before the actual bonding by rolling is achieved at the same time, so that shifting and slipping of the layers during the rolling is effectively avoided and there is no need for an additional holding device.
Pursuant to a further advantageous embodiment, the fibers are surface-treated with zinc, nickel or copper or a different wetting-promoting surface layer. The formation of the alloy during bonding of the sandwich structure is additionally controlled in this way by simple means.
The inventive composite material can be further processed into semi-finished products of all types. It is used mainly in aircraft technology, for example, for fuselage lining and for the outer shell. In addition, it can be used for other vehicle applications in military technology as well as for manufacturing transport containers.
In the following, the invention is explained in greater detail by means of the enclosed Figures.